Aerosols, that is, suspensions of fine particles in air or gas, are today recognized to play a central role in diverse environmental problems such as climate change, impaired visibility caused by the particles in the atmosphere and eutrofication of unhabited lands. They also have an effect to respiratory diseases. Aerosols are also the sites on which heterogeneous reactions of gaseous trace constituents occur. The sources of each of the major chemical constituents of the aerosols must be known and their role in atmospheric processes must be elucidated, in order to regulate and reduce their detrimental effects. There are indications that most of the mass in the fine aerosols (Dp.<2.5 μm) is secondary, i.e. is not directly emitted, but formed from gaseous precursors in the atmosphere. The absence of reliable aerosol data prevents understanding of the formation of these secondary aerosols and evaluation of their treatment in chemical transport models. However, legislation will soon be needed to consider the sources and contributions of the major individual aerosol components to the total mass in order to develop efficient abatement strategies for preventing climate change.
Detection and analysis of aerosols using a condensation particle counters (CPC), often known as condensation nucleus counters (CNC) is well known. CPC is used for example to detect small particles in aerosols, for example for outdoor and indoor air-quality research, filter and air cleaner research, particle formation and growth studies and combustion and engine-exhaust studies. The CPC is also used as the primary detection for obtaining particle size distributions, for example in scanning mobility particle sizers. With CPC it is possible to detect particles as small as 3 nm in diameter. CPC detects particles by condensing a vapor on the particles to grow them to large enough size that they can be counted e.g. optically or by other means. The measurement usually involves four steps: 1) the production of sufficient quantities of vapor, 2) creation of supersaturation necessary to activate the particles, 3) maintenance of the particles in the supersaturated state long enough to grow them to a detectable size and 4) detection of the grown particles. In a CPC, the aerosol is first saturated with a vapor and subsequently cooled to induce the supersaturation conditions. For a given saturation ration, the vapor can condense onto particles only if they are large enough. The minimum particle size capable of acting as a condensation nucleus is called the Kelvin diameter. The relationship between the supersaturation rate and Kelvin diameter (dp) can be expressed as the following function [Tang 1976, Friedlander 1977]:
            P      d              P      s        =      δ    ⁢                  ⁢          m      f        ⁢          exp      ⁡              (                              4            ⁢            v            ⁢                                                  ⁢            γ                                RTd            p                          )            Where:                pd is the saturation vapor pressure on particle surface        ps is the equilibrium saturation vapor pressure        δ is activity coefficient        mf is mole fraction of the solute        γ is the surface tension of the liquid        ν is the molar volume of the liquid        R is the gas constant and        T is the temperature.        
Equation is derived for vapor condensed on a liquid droplets of the same material or on insoluble particles with wettable surface properties for the working fluid.
FIG. 1 shows a CPC according to prior art, which is a so-called cooling-type CPC. In this example, the vapor needed for particle growing is produced through cleaning and supersaturation from aerosol gas itself, and it is called here the sheath flow. In other words, the sheath flow is filtered aerosol flow, i.e. it contains the same gas mixture as the sample flow, without solid particles. The main parts of the CPC are a flow divider 5, saturator 1, condenser 2 and detector 7. The flow divider 5 divides the aerosol flow to sheath flow and sample flow. Saturator 1 has two sections which are integrated together: a saturating section 1a, which is a heated tube, with liquid impregnated felt lining, where the sheath flow becomes saturated with vapor, and a heated section 1b, without felt lining. In the condenser 2, the vapors condense on the particles contained in the sample flow, thus enlarging them to become big enough to be detected by an optical detector 7. The condenser 2 is cooled to a temperature lower than the temperature of the saturator 1.
The flow divider 5 comprises two channels, for example pipes or the like, an inner channel 5a and an outer channel 5b, which are set within each other. The inner channel 5a is attached at it's other end to channel 4 for taking in the aerosol flow and it's other end extends inside the saturator heated section 1b. The outer channel 5b forms the outer surface of the flow divider 5 and it is dosed from the top, that is, from the end inside the saturator heated section 1b, with a cover or the like, the center of which cover is permeated by a sample flow capillary 3.
The intaken aerosol flow flows upwards in inner channel 5a. The inner channel 5a ends at a distance from the cover, thus forming a slit 8 between the upper edge of the wall of the inner channel 5a and the cover. Part of the aerosol flow in the inner channel 5a enters through the slit to the space between the inner channel 5a and the outer channel 5b to form a sheath flow. The sheath flow is taken out from the flow divider 5 and led onwards for cleaning with filter 6. The filter 6 removes all particles from the sheath flow, after which the cleaned sheath flow is directed to a saturator 1. The flow divider ends to a sample flow capillary 3, which extends in to the saturator heated section 1b. 
The sample flow, which has been separated from the aerosol flow by means of a sample flow capillary 3 attached to the flow divider 5, enters the cleaned and vapor saturated sheath flow in saturator heated part 1b, mixes with the saturated sheath flow and flows to the condenser 2. In the condenser 2, the vapors condense on the particles contained in the sample flow, thus enlarging them to become big enough to be detected by an optical detector 7 following the condenser 2.
However, there are several problems with the CPC according to FIG. 1. The detector responses are slow, and turbulent mixing of flows causes supersaturation fluctuations and fluctuations of cutsize both in saturator and in condenser. Another problem is that flow recirculations are created in the system. Thus, while some grown particles immediately exit the condenser and enter the detector, other particles just circulate inside the condenser and randomly exit at some later time, introducing an exponentially decaying distribution of delays between the time particle enters condenser and when it is detected. Thus it is impossible to obtain sensitive size distributions. Also due to long condenser which is needed for particle growth, the ultra small particles in the flow tend to hit the condenser walls, prior reaching the detector, thus causing erroneous results. Further problem is the long saturator and the large diameter of the felt tube in the saturation section 1a. This causes a laminar sheath flow in the felt tube of the saturation section 1a, resulting in a non-homogenous formation of vapor.